The present invention relates to paper coaters generally, and in particular to an improved doctor assembly for a dip roll coating system, and a method of coating paper in which film split and streaking patterns in a coating applied onto a paper web are significantly minimized.
Paper coating processes continue to be performed at faster web speeds to increase productivity. At the same time, paper quality continues to rise because of printer demands, necessitating increasingly higher coat weights to produce paper of the required quality. The standard paper coating system for high coat weights and high web speeds has been the dip roll coater. Essentially, to apply a high weight of coating onto the surface of a paper web traveling at a high rate of speed, a dip roll applicator applies an excess of coating onto the web as it is carried through a nip between the dip roll and a backing roll. Downstream from the dip roll, the coating is doctored on the web by a blade.
Dip roll coating application has long been plagued by narrow streaky coating lay caused by film splitting that occurs as the dip roll rotates away from the web on the outgoing side of the backing roll/dip roll nip. At high web speeds, the dip roll produces a severe film split pattern in the excess coating layer applied onto the web, i.e., separations or thin areas occur in the coating, extending along the direction of web travel. The narrow coating bands created at the backing roll/dip roll nip travel on the coated web to the blade, and as coater speeds increase, especially above 2500 fpm and faster, the film split pattern becomes unacceptably severe. When this nonuniform layer of excess coating impacts the blade, it exerts varying impulse forces against the blade and is doctored nonuniformly. In consequence, the film split pattern is not completely removed by the blade and appears as narrow machine direction banding in the finished sheet, and the higher the coat weight, coating material viscosity and web speed, the more pronounced is the film split pattern. In addition, when the excess coating impacts the doctor blade, some spatters back and lands on the web upstream of the blade. This turbulent spattering becomes more violent as coater speeds increase, producing blotchy patterns that are visible in the finished sheet.